Allow flashing EC only, update docs on flashing

[crawfxrd]

As somewhat of a hack, allow flashing only the EC by deleting the firmware.rom file before running flash.sh.

flash.sh was changed from internally flashing to using the UEFI application (firmware-update). The option --without-ec was removed as part of this.

[XV-02]

Are there any specific behavioural changes I should be looking for on this?

It looks like:

• I can successfully independently run the flash cycle with either ec.rom or firmware.rom deleted.
• The firmware lock functions as I would broadly expect ( 1- I used the internal flasher; 2 - on reboot, firmware update presents the "hit enter to flash" prompt; 3 - on hitting enter, system reboots and presents the unlock screen; 4 - I unlock and system flashes as expected.)

I do note that, on a clean pop install and fresh repo clone, running the build script after ./scripts/install-deps.sh returns an error around pkg-config and libcrypto that required installing libssl-dev to seemingly fix.